G-M tubes are used to detect ionising radiation and in particular may be operable to detect electromagnetic radiation (.gamma.-rays) resulting from the decay of radio-active material, for example in the energy range of 50 keV-1.3 MeV. The sensitivity of an unshielded G-M tube, typically expresses as the number of counts per roentgen, varies significantly with energy within this range, for example from around 400 keV downwards and especially below about 200 keV.
It is known to provide an energy filter aobut a G-M tube to reduce the variation of sensitivity of the tube with the energy of incident .gamma.-radiation. A filter known from the paper "A Geiger-Muller .gamma.-Ray Dosimeter With Low Neutron Sensitivity" by E. B. Wagner and G. S. Hurst, Health Physics, Vol. 5, pages 20-26 (1961) comprises two successive annular layers respectively of tin and lead around the tube (which, as is usual, is elongated and substantially rotationally symmetrical) and two successive discs respectively of tin and lead abutting the annular layers adjacent one axial end of the tube, these materials being mounted within a synthetic plastics (fluorothene) jacket. This arrangement is said to make the counter (Philips type number 18509, now available as Mullard type ZP 1310) furnish readings of exposure dose in roentgens that are essentially independent of .gamma.-ray energies down to 150 keV; a graph in the paper indicates a falling response from about 300 keV downwards.
Other known filters, proposed for use with Mullard (registered Trade Mark) G-M tubes, each comprise two longitudinally-separated annular bodies about the tube and a disc adjacent one axial end of the tube; the disc is separated by a gap from the adjacent annular body, and for tubes having a protrusion at that end, has a central aperture into which the protrusion extends. The disc consists of tin, and the annular bodies consist either of tin or of two layers respectively of ten and lead. As in the filter first mentioned above, the energy-absorbing elements of the filter are mounted in a synthetic plastics jacket. The surfaces of the annular bodies bounding the gap therebetween are inclined away from each other at an angle to the longitudinal axis of the tube varying (from one filter to another) from 70.degree. and down to 45.degree..
In a combination of a filter and a G-M tube fitted therein available as Mullard type ZP 1311, the filter consists of two identical, longitudinally spaced bodies of tin, each comprising an annular portion and, contiguous with one end thereof, a disc portion with a central aperture. The adjacent surfaces of the annular portions bounding the gap between the two bodies are curved substantially in the form of a quadrant of a circle.
Yet another filter is known from published U.K. patent application GB No. 2 097 640 A. This filter comprises a copper sheath and attached thereabout a discontinuous jacket of a 60/40 tin-lead alloy in the form of two axially-spaced rings and one disc at one end of the sheath, the disc being spaced from the adjacent ring. The surfaces of the rings which define the annular gap therebetween are depicted as being inclined away from each other at an angle to the longitudinal axis of the tube of about 60.degree..